Reset browser.check_default_browser to true when Chrome is the user's default browser

BUG=492734

Review URL: https://codereview.chromium.org/1182513007

Cr-Commit-Position: refs/heads/master@{#334725}
(cherry picked from commit b0e8c81ed1be3462355c5d5543d2636545029d63)

TBR=grt@chromium.org

Review URL: https://codereview.chromium.org/1176423009.

Cr-Commit-Position: refs/branch-heads/2357@{#475}
Cr-Branched-From: 59d4494849b405682265ed5d3f5164573b9a939b-refs/heads/master@{#323860}